If
you are an audiophile, you probably have spent a lot of effort setting
up your home entertainment system, car system, and maybe even your portable
music player. You know all about how to select speakers, where to place
them, the kind of wire and connectors to use, treble, midtone, and bass
equalization. You know what you like, but the way you perceive sounds
is not the way many other people do. Even if your hearing follows a
'normal' response curve per standardized audio tests, personal preferences
vary widely in part because the way brains are wired (speed of sound
processing, preferred musical style, state of mind - maybe you're insane)
and in part because of the impact of sound waves upon your body (soft
tissue and bone pressure wave conduction, clothing worn). Pitch, timbre,
tone, and loudness has been extensively studied and quantified. While
three of the four are purely objective measurements, loudness is the
one that is subjective. Loudness, simply put, is the level of amplitude
by which frequencies other than a chosen standard (e.g., 1 kHz)
must be increased or decreased to be perceived to have an equal level
of "loudness." Research performed by Harvey Fletcher and Wilder Munson
in the 1930s used a panel of human subjects to construct a set of curves
(Fletcher-Munson curves) of equal loudness based on frequency and the
decibel level of the reference tone. Being a lover of nice music but
stuck at a relatively low income level, I never invested in really good
sound equipment; my experience with sophisticated equalization consisted
of deciding when or when not to push the "Loudness" button on the front
panel of my $50 Radio Shack stereo unit. Now at least I know what it
did. This article goes into great detail on the concept of loudness
and how stereo systems implement compensation.

Why Loudness Control?

By Norman H. Crowhurst

A
fresh approach to the need for a true Loudness control and information
on its proper adjustment.

Any subject that depends upon appreciation
by the human senses is bound to be one that comes up for perennial discussion.
This is one of them. There are two extreme schools of thought: one of
these suggests that correct reproduction requires an accurate re-creation
of the original sound wave pattern; this means that all of the components
in the original program must be reproduced in exact proportion and at
the same level; this philosophy does not allow of anything so intangible
as a loudness control.

At the other extreme we have the people
who believe that human hearing has been "scientifically" established
by such data as the Fletcher-Munson curves; therefore, to give a true
subjective impression of any reproduced program material, a loudness
control must be incorporated that introduces compensation according
to the accepted and proven data published first by Fletcher and Munson.

In between these two admittedly extremist views a number of
subsidiary questions arise and confuse the novitiate still further.
Hence the perennial topic. Let's start out from the classic point of
argument that faithful reproduction should be an exact re-creating of
the original sound wave pattern.

Pursuing this ideal to its
logical conclusion one would assume the loudspeaker should generate
a sound intensity in the listening room identical to that received by
the microphone in the recording studio. This leads us to question number
one.

When most people say this, they subconsciously think of
listening to an orchestral performance in a typical auditorium, from
a typical auditor's seat. But, if a microphone were placed in the position
of the typical auditor's seat and a recording made, no one with any
musical appreciation would want the disc. The reproduced sound would
be quite unlike the impression conveyed when sitting in that seat personally.
Why should this be?

Whether we use a stereophonic system for
reproducing our sound or not, our hearing is always binaural, whereas
the microphone is not, placed in the position of the typical auditor's
seat or wherever you will.

Some readers of this magazine have
the misfortune to be deaf in one ear. To them, the remarks which follow
will seem incomprehensible, except as pure theory. A few years ago the
author was deaf in one ear for a period, an experience which enabled
him to appreciate how different everything sounds this way. This experience
also confirmed the following explanation.

When we take up our
seat in the auditorium and listen to the program, we are primarily conscious
of the orchestra; secondarily we hear the reverberation coming from
other parts of the building, as a kind of echo, although the time interval
is usually too small for it to be noticeable as a separate entity. But
our binaural capacity for differentiating directions enables us clearly
to separate, in our subconscious perception, the original sound from
the orchestra and that due to the same program reverberated around the
auditorium.

The difference in intensity between these two components
of sound may not be more than 10 or 20 db but, because of our subconscious
interest in the program itself, attention is focused on the orchestra
and the reverberant sound is heard merely as a background that lends
character. Because of this, our subconscious subjectively increases
the apparent difference in level to much more than the actual 10 or
20 db.

The microphone, however, being a much more non-subjective
device than a pair of human ears, would pick up the whole sound received
as a conglomerate, with only the actual 10 to 20 db differential between
the direct sound from the orchestra and the reverberant sound from various
parts of the auditorium. When this sound is reproduced over any system
whatever, even in a thoroughly acoustically damped room, it will sound
extremely reverberant and "echoy." This is because now the direct and
reverberant sound both come from the same source, and our binaural faculty
of hearing no longer can go to work on it in the same way and direct
attention to the orchestra exclusively.

Recording studio personnel,
of course are thoroughly conversant with this fact. They don't advertise
it, because listeners naturally prefer to think the recording transports
them to a position in a typical listener's seat in some auditorium.
But, to get an effect that puts the direct sound and the reverberant
sound more in the correct proportion, as the listener thinks he would
hear it, the microphone must be placed very much nearer to the orchestra,
or, what means practically the same thing (for this purpose), a highly
directional microphone must be used to favor sound from the orchestra
more than the reverberation.

Modern practice usually places
the microphone quite close to the orchestra, or uses an array of microphones
distributed among the orchestra so as first to get a pickup that represents
the original program material, exactly as played by the orchestra, practically
without any reverberation at all. Then, if some reverberation effect
is required, the studio uses an echo chamber to add this artificially.
This procedure enables the effect to be easily controlled so the resulting
sound will have just the right amount of reverberation to give the desired
effect.

Certainly this technique produces very fine sounding
records, judging by recent releases from most of the record companies.
Even the sticklers for accurate reproduction of the original sound will
have to admit this although, if they knew it was made this way, they
would call it "phony." But let's ask them, which original sound do they
really want reproduced accurately, the sound of the orchestra, as heard
by, say the conductor, or one of the microphones scattered throughout
the orchestra, or the sound as it might be heard in the echo chamber,
somewhere between the loudspeaker that produces the sound and the microphone
used to pick up the "echo" ?

It is quite obvious that neither
of these sounds will be the same as that we hope to hear in our living
room.

When you sit in the average seat in an auditorium and
listen to an orchestral concert, the sound you hear is quite different
from that heard by the conductor, or by any of the instrumentalists
in the orchestra. All of these performers work to give the best impression
to you, the paying audience. The same is true making recordings, only
here we have a few more "performers", like the engineers who operate
the various controls associated with the microphone levels, echo chamber,
etc.

The important thing is that the intended impression is
put across, whether by the individual performers working collectively
in the orchestra in an actual auditorium, or whether by all those who
work together to make a satisfactory record.

Fig. 1. The Fletcher-Munson curves of equal loudness at various
listening levels.

Picking up the program at close quarters like this gives another advantage
to the record maker. It assists in getting a better dynamic range onto
the recording. As well as giving he direct sound from the instruments
a bigger advantage over reverberant sound reflected around the studio,
it also gives the program material a better advantage over various other
stray sounds, noises that can creep into the studio by devious means.

A good studio is built with all kinds of sound insulation to
keep out extraneous and undesired noises. But all such devices only
attenuate the undesired sounds, they don't completely eliminate them.
Consequently, it is still advantageous to have a fairly high level of
sound at the microphone, so as to give the biggest possible margin over
unwanted background sounds.

This also makes it easier on the
recording system, because it gets the program material that much farther
above background noise from the microphone itself and the amplifier
system.

But picking up sound for the record at this high level
and recording it through a system with standard equalization characteristics,
means that the original sound pattern can only hope to be truly re-created
when played back through the correct compensating playback equalizer
characteristic and reproduced over a loudspeaker system at the original
sound intensity. This will be too loud for comfort - much louder than
one would receive sound in the average seat of an auditorium. The intensity
of sound in the living room, reproduced this way, would be comparable
to the level of sound in the area occupied by the orchestra itself.

To get the true perspective of the music. one needs to be a
little farther away or to hear the sound at somewhat lower level. Consequently
it is usually desirable to reproduce the sound at a somewhat lower intensity
than it is picked up at the microphone for recording.

Let us
now consider another angle. Live musicians can, and do, provide program
music for any of the purposes for which we use reproduced music. As
well as playing to an audience in an auditorium, musicians, upon occasion,
play to entertain people in a living room, or to provide background
music in a restaurant or club, while the occupants talk. A group of
musicians playing under any of these conditions will naturally adjust
their performance to suit the purpose in hand. They will play louder
in an auditorium and much quieter in a restaurant or club. How do musicians
themselves make this loudness adjustment?

This does not mean
that each instrumentalist will reduce his volume by a precise number
of decibels. Rather, each reduces his own loudness so that the same
sense of balance is achieved at the lower loudness level. This, naturally
enough, is done according to the judgment of the musicians' ears.

While musicians' hearing certainly is differently conditioned
from that of most of us (which is what we infer by crediting a person
with a "musical ear"), there is no intrinsic difference between the
ears of musicians and those of the rest of the population. The famous
Fletcher-Munson loudness contours were based on the hearing of musicians
as well as of other groups of people. So one would expect an average
musician to have a sense of comparative loudness similar to an average
person of any other group.

As this is the case, each instrumentalist
will automatically adjust the loudness of his playing so as to obtain
a perspective of over-all balance that agrees with the general pattern
of human loudness sensation.

Fig. 2. Comparison between range of control afforded by the
average tone control and the kind of change in response required
for satisfactory loudness control.

If we base this on the experience represented in the Fletcher-Munson
contours, shown in Fig. 1, and assume that the difference in loudness
from the concert hall level to the background music level is from the
70 curve to the 40 curve, this will be a difference of 30 decibels at
1000 cycles and also at most frequencies above this. So musicians playing
instruments with frequencies from 1000 cycles and up - or even 500 cycles
and up, will reduce the intensity of tone from their instruments by
about 30 decibels, or a power ratio of 1000 to 1. But, going down to
instruments like the string bass, which may be playing tones in the
region from 40 to 80 cycles (taking 60 cycles as an average), the difference
between the curve marked 70 and the curve marked 40 is only about 12
decibels.

While the bass player will un­doubtedly reduce the
apparent loudness of his instrument by the same amount as other players
in the orchestra, the actual intensity difference is much less. The
higher instruments reduce by 30 decibels, while the bass player only
reduces by 12.

As we have seen in the preceding discussion,
music is never recorded at such a low level. If you want to use recorded
music for this purpose, you will have to turn the gain or volume control
down by 30 decibels or so, and if you want to play at a level suitable
for the average living room you will still need to turn it down, by
possibly 12 or 15 decibels, from the intensity at which the music was
originally recorded.

But to retain the balance at which the
group of musicians would naturally play, the bass frequencies must be
turned down to a lesser extent than the middle and higher frequencies.
This is our argument for using a loudness control. If an ordinary volume
control is used (which should more accurately be termed a gain control),
to produce this effect, the balance is disturbed.

The effect
produced is not that of the orchestra playing more quietly but of its
being farther away.

This is because all frequencies are attenuated
in proportion, which is exactly what happens when you listen to a musical
program from a distance, unless of course there is exceptional transmission
through floors and ceiling, such as may occur if the "distance" is from
one floor to another of an apartment house!

This has about sated
the esthetic factors that form the basis for discussion as to whether
the loudness control is needed at all and, if so, what kind of control
should be provided. Having settled that we do need control facilities
to make this adjustment, the next point is if a special loudness control
is needed, or whether an ordinary volume (or gain) control can be used
satisfactorily in conjunction with the regular tone controls?

The alternatives in this stage of the argument are: whether we have
a loudness control that automatically increases the relative amount
of bass, measured by intensity, as the volume goes down, or whether
we use a "straight" gain or volume control and then use the bass knob
of the tone controls to get more bass when working at a lower loudness
level, if necessary also giving a little boost to the treble end for
the same reason. Don't both of these methods really achieve the same
thing? And, using just a gain control with tone controls, surely, is
simpler than having to "bother with" an additional loudness control?

Time was when it was argued that equalization facilities were
not required for all of the different recording characteristics - the
tone control would serve for this purpose as well. Going back still
further, there was a time when tone control consisted of just a variable
resistance in series with a capacitor, conveniently placed somewhere
in the circuit, which produced only a variable treble cut. There was
no true bass control. All too often, this control was operated with
the treble fully cut, because this removed a maximum of unwanted background
noise, scratches, and plops, either due to scratch on the record or
due to static coming in on the radio. The average listener of those
times couldn't tell the difference between the maximum and minimum position
of the control, as far as the quality of the program itself was concerned.
The only effect noticed was the apparent reduction of background noise.

But, since then, listeners have become educated to high fidelity.
And the fidelity, both of equipment and of recordings, has improved
tremendously. Nowadays, the comparative novice in high fidelity can
tell the difference between bass boost and treble cut, or vice versa.
It requires a little more education to discern the difference between
records played with the correct equalization characteristic and with
the wrong one, but it certainly is possible to tell the difference.

But, a decade or so ago, strong arguments were put forward that
the tone control should serve this function in addition to any other
tone compensation that may be desired. Nowadays, any self-respecting
preamplifier provides equalization characteristics for RIAA, and usually
one or two other recording characteristics that may be encountered;
and the average high-fidelity listener appreciates this position and
reserves the use of tone controls for their proper function - adjusting
for differences in balance that may become apparent, due to differences
in the acoustic characteristics of the studio and the listening room,
or due to any other variations in the balance of musical composition
of the program.

Now the extremist comes along with the loudness
control and says this also is a must, in addition to tone controls and
equalization. Are we going to accept this further stage as a necessity,
along with equalization, or is this really a luxury that is completely
unnecessary?

This would seem to depend upon the accuracy of
your perceptive powers. This can be illustrated at Fig. 2, which shows
a comparison. between the range of control given by the average tone
control and the kind of change in response needed in a loudness control,
to follow the variation predicted by the Fletcher-Munson curves. For
the tone control to get enough boost in level at the low-frequency end,
gives too much boost at frequencies higher up. The boost has to start
in the region of 500 cycles. whereas the loudness contour shows not
very much difference at 500 cycles based on true loudness contours.

Fig. 3. Tapped pot used as loudness control.

As the region in question is from 100 to 500 cycles, which corresponds
to the middle register of music, use of the low-frequency boost in conjunction
with the volume control will over-emphasize the middle register which
is not at all what is required for true loudness impression. So the
average tone control puts the low-frequency accentuation in the wrong
place, and by the wrong degree.

If you can hear this difference.
which an educated high-fidelity listener certainly can, then a loudness
control is definitely an advantage, in comparison with the use of a
volume control with separate tone controls.

The next question
is, what arrangement of loudness control and gain or volume control
should be used? Do we need both controls, or is one control that can
serve both purposes enough?

Some preamplifiers have appeared
with a single control, and a loudness volume switch, which alters the
function of the control from one to the other. A little thought will
show that this achieves practically no advantage over an amplifier provided
with only one or the other.

To be effective, a loudness control
must produce the right contour appropriate to the level at which the
program is played. As different programs are often recorded at quite
widely differing levels (as regards the input delivered to the preamplifier),
there is no guarantee that setting the volume/loudness dial to a specific
position (say 5) will always reproduce a program at the same loudness,
regardless of what disc is being played. But this is how a loudness
control should operate, otherwise the loudness contours will not come
in the right place.

What we need then is two separate controls:
a gain or volume control, to adjust the input received from the recording,
so that setting the loudness control to a particular position will give
the right loudness in the room for that setting. For example, if you
normally listen at a level corresponding to 50 in the loudness contour
family, the loudness control should be set to a position which makes
this correction from the average recording level of 70. Then the gain
control should be adjusted on each particular recording, so the loudness
in the room corresponds with the average to which you have become accustomed.

Then, once you have set the gain control for this particular
degree of loudness, you can alter the loudness control to play the music
softer or louder. Of course, you do not always have to go through this
routine just to get it right for a particular recording. It is much
simpler to set the loudness control for the desired loudness. This you
will become used to with a little practice. Then adjust the gain control
so the actual loudness corresponds with the setting on the loudness
control.

Some loudness controls, instead of providing a continuously
adjustable range, use a multi-position switch, that may have three positions
for example, marked low, normal and high, referring to the degree of
loudness. This control can then be set according to how you want the
music to play in the room and the normal volume control can then be
used to get the loudness right. This will generally give a pretty close
approximation to the right loudness contour and from this point the
tone control can be used to adjust for slight deviations in balance,
if it does not sound to the best advantage.

The switched arrangement
has the advantage that the correct contour can be tailored into the
circuit. Some loudness controls use just a simple tapped potentiometer,
the schematic of which is shown in Fig. 3. This does not give sufficient
steepness to the loudness compensation, particularly for playing at
low levels. In fact, it is little better than the use of the tone control.
Maybe, if you have a separate tone control and a loudness control of
this type, the two can be used together to get results that come nearer
to being right than just the tone control by itself.

But the
best arrangement, if you have a loudness control that is continuously
variable (like most volume controls), is to use one which comes as a
ganged potentiometer, two or more decks with associated components wired
on to the potentiometers to produce the required variation of loudness
contour as the control is turned.

But discussion of the precise
way of getting the right characteristics into a loudness control is
another article altogether. It must be said that, after becoming accustomed
to the correct use of a good loudness control, going back to a system
that only has the regular volume and tone controls seems to lack the
degree of smoothness in reproduction obtainable with the loudness control.
Especially is it difficult to get satisfaction for low level listening,
which is where the good loudness control really does its job well.

This can easily be demonstrated on a preamplifier that has both
forms of control, playing a good wide-range program that has plenty
of bass and treble. Turn down, first of all, the loudness control and
the effect is that the orchestra or program is being played more quietly,
but still in the room with you. Now restore the loudness control to
the maximum comfortable level and turn down the volume control. This
time the effect is not of the program being played more quietly in the
same room, but it seems to go farther away as if it is no longer in
the room with you but playing from somewhere down the street.

Sometimes it is nice to have both of these facilities available,
but if you principally want to present music that sounds good, you certainly
want to have it sound as if it is in the room with you and not being
played from a distance away. So it looks as if we really need loudness
controls.

Just a final word, however. This decision is not going
overboard on the second extreme mentioned at the beginning of this article.
The Fletcher-Munson curves may be (and are) scientific, but they do
not represent the exact loudness contours of your ears. In their investigation,
it appeared that individual hearing is so divergent that it is extremely
unlikely that anyone has average hearing. But use of these
average curves assures a control that should satisfy.

Although
each of us has hearing that diverges one way or another from these curves,
they represent the shape of almost everyone's hearing characteristic.
This means that our individual loudness difference contours will be
very close, although the actual contours deviate widely. So a well-designed
loudness control, based on loudness differences, will sound
right almost anyone.